This invention relates to automated working of metals and, more particularly, to a method for the working or final sizing of holes, for example, holes in aluminum, steel, magnesium, and alloys used in the structure of aircraft and aerospace vehicles.
It has long been the practice, in the fabrication and assembly of structures, such, for example, of aircraft and aerospace structures, to first drill or punch holes through the metal through which bolts, rivets, or fasteners are to pass and to then work the metal around the hole. Such working stretches the workpiece beyond the yield strength of the workpiece and increases the fatigue life of the structure about the hole by greatly reducing flaws or stress concentration points and by generating compressive stress gradients within the material about the worked hole. Because the greatest tensile stress concentration from external loading occurs at the hole surface, increased residual compression stress gradients increases the fatigue life of the structure.
Prior to the invention disclosed in U.S. Pat. No. 4,433,567 which issued to the present assignee, one method followed in working of drilled or punched holes has been to force a ball having a diameter slightly larger than the hole through the hole, enlarging the hole and compressing the metal around the hole. Rather than using a ball, slotted collars or split sleeves and oversized mandrels have also been employed. In this arrangement, the slotted collar or split sleeve, having a relaxed diameter which will fit through the drilled hole, is placed in the hole, expanded with a slightly oversized mandrel and the mandrel is removed. The slotted collar or split sleeve springs back to its relaxed diameter, is removed from the hole and discarded.
In the instance of the oversized ball and the slotted collar or split sleeve and the oversized mandrel, in addition to compressing, stressing and increasing the strength and fatigue resistance around the hole, the oversized ball and collar or sleeve, where such collar or sleeve is used, distort or displace the metal. The overized ball tends to force the metal through as well as radially outwardly of the hole. Thus, the ends of the hole wall are not square and must be subsequently machined or finished. In the case of the slotted collar or split sleeve, a ridge in the hole wall at the collar slot or sleeve split may be formed which must be removed by machining or finishing. Machining or finishing to square the hole wall or remove the ridge in the wall adds to the cost and, to a degree, reduces the advantages attained by working. The likelihood of fatigue related failures is increased.
With the invention of the mentioned patent, it has been discovered that working of holes can be substantially improved and the need for subsequent machining or finishing eliminated resulting in the cost being substantially reduced. It has been discovered that, by employing a full sleeve of a shape memory material, preformed and pretreated in a particular manner and by inserting such sleeve in the hole to be worked, the sleeve can be plastically expanded with a mandrel such that the metal around the hole is worked and compression stressed and the sleeve can be brought thermally to a critical temperature where it shrinks and can be reused. Machining or finishing of the resulting worked hole is not necessary and the collar or sleeve may be plastically expanded and reused again and again by plastically expanding and thermally treating it to cause the sleeve to shrink to its memory shape.
In the prior invention the full sleeve to be used in working is fabricated from a metal treated to have a memory shape, such as Nitinol nickel-titanium alloys or copper-zinc-aluminum alloys, and the like.
Nitinol is a family of nickel-titanium alloys having a chemical composition of from about 53% to 57% by weight of nickel and the balance titanium with small percentages of other elements added to adjust transformation temperature, e.g., iron or cobalt. It was developed by the U.S. Naval Ordnance Laboratory. Copper-zinc-aluminum alloys having about 70% by weight of copper and the balance zinc with small additions of aluminum have also been developed.
The prior invention instituted use of a sleeve of a shape memory alloy, having a transition temperature range between -320.degree. F. and +300.degree. F. which is given a memory shape. It is then cooled below the transition temperature and inserted into the hole and expanded while still below the transition temperature. A mandrel having a diameter larger than the I.D. of the sleeve, which diameter combined with the thickness of the expanded sleeve makes up the diameter of the hole being worked, is inserted into the sleeve while in the hole and is forced through the sleeve. The mandrel passing through the sleeve further enlarges the sleeve and works the hole. The sleeve is then warmed up to above its critical transition temperature and returns to its shrunken memory shape which, of course, is of a smaller O.D. than the I.D. of the worked hole, and is then removed from the worked hole.
After each use, the memory shape sleeve, at its memory shape, can be cooled below its transition temperature, inserted in a hole to be worked, enlarged with a mandrel forced through the memory shape sleeve, warmed above the critical temperature and shrink to its memory shape and the cycle can be repeated for the next hole to be worked.
The memory shape sleeve is continuous. Thus, there is no ridge to be removed nor edge to square. The sleeve is not discarded. Working is simplified and costs are reduced.